1. Field of the Invention
This invention relates to a solar heating system, and particularly to one including a controller having an equilibrium curve for the system that enables the controller based on temperature and sunlight conditions to determine if the system can gain or lose heat.
2. Background Discussion
In solar heating, a pump circulates a heat transfer medium in a reservoir through a solar collector exposed to sunlight. Temperature sensors sense the temperature of the heat transfer medium, commonly water. The problem is that these temperature sensors are inaccurate. Consequently, the pump is frequently not activated even though the system can gain heat. Conversely, the pump often is activated when the system shall lose heat.
The pump operation is based on the measured temperature difference between the water in the collector and the water in the reservoir. When the water in the collector is at a higher temperature than the water in the reservoir, a controller for the system turns the pump on and circulation commences. When a positive measured temperature difference can no longer be maintained, the controller turns the pump off. This approach does not work well if the measured temperatures are not the same as the actual temperatures.
It is common for the output of the temperature sensors to drift due to electronic errors, thermal cycling, and environmental degradation. The problems with conventional solar heating systems are discussed in a number of publications by the Solar Energy Research Institute of the Department of Energy, including:
1. Evaluation and Laboratory Testing of Solar Domestic Hot Water Control Systems (SERI/TR-254-1805, February 1983), and
2. Reliability Testing of Active SDHW Components, Part I: Test Results of Sensors Used in Control Systems (SERI/TR-253-2602).
Thus, an uncertainty exists in the accuracy of the measured temperatures.
To compensate for this uncertainty, the industry introduced bias terms (TB1 & TB2) used in programming the operation of the controller. Typical values for TB1 and TB2 are between 2 and 15 degrees Fahrenheit, for example, TB1 equals 8.degree. F. and TB2 equals 3.degree. F. The measured temperature difference must be greater than TB1 before the pump is turned on. When the measured temperature difference is less than TB2, the pump is turned off. This technique assumes that the drift of the sensors never exceeds the value of TB1 or TB2. The problem with using bias terms is that pump cycling occurs and that over time the drift in the sensors sometimes exceed the bias terms.
Some performance factors inherent in control systems which use measured temperature differences are worth noting. During start-up conditions, the system waits until the required value of TB1 is reached, at which time it turns the pump on. The temperature of the collector at this time was reached under stagnant conditions (no flow). It had taken from sunrise until that time to reach that temperature difference. When the pump turns on, the sunlight intensity will not be high enough to maintain the temperature difference of TB2 under non-zero flow conditions and so the pump will turn off. The hot water that was in the collector is now waiting in the return line between the collector and the reservoir where its heat losses are high due to the high surface to volume ratio of the return line. Cold water that was in the supply line between the reservoir and the collector is now in the collector gaining heat. Hot water that was in the reservoir is now cooling down in the supply line. On/off pump cycling occurs until a measured temperature difference greater than TB2 can be maintained or exceeded. A similar situation occurs in late afternoon when the sunlight intensity is decreasing and a measured temperature difference greater than TB2 can not be maintained, even though there is still a net heat gain potential. Under certain conditions, this mode of operation can actually lose heat when it should be gaining heat.
On/off cycling induces unnecessary duty cycles on the pump, the electronics which turn the pump on and off, and any valves in the system. Each time the pump turns on, high electrical current transients pass through the windings in the pump motor due to its natural inductance. This increases operational costs significantly. Transients of this nature shorten the pump life and the life of the electronic components which deliver the power. Valves with plastic or rubber seats suffer excessive wear, shortening their useful life. To diminish this problem, the industry introduced proportional control which varies the water flow rate with the measured temperature difference. However, a non-zero value of TB1 and TB2 must still be used due to the sensor errors.
It is important to maintain turbulent flow throughout the collectors so that heat transfer efficiency is optimized. Although proportional control diminishes the on/off cycling problem, it does not eliminate it and the system may operate with laminar flow much of the day instead of fully turbulent flow. Laminar flow can not remove heat from the collector as efficiently as turbulent flow, causing the system to operate at an efficiency level lower than it could. Proportional control systems can only be used on small fractional horsepower pumps. Pool and commercial solar systems use large pumps (1 to 10 horsepower).
Government tests and surveys as reported in Evaluation and Laboratory Testing of Solar Domestic Hot Water Control Systems (SERI/TR-254-1805, February 1983) indicate that the mean time between failure for control systems, which use measured temperature differences, is 4.1 years. The causes are:
1) If the drift of the collector temperature measurement is positive and/or the drift of the reservoir temperature measurement is negative, the measured temperature difference will be greater than the actual, causing the system to turn on too early in the morning and turn off too late in the afternoon. Eventually, the pump may remain on even after sunset. Conversely, if the drift of the collector temperature measurement is negative and/or the drift of the reservoir temperature measurement is positive, the measured temperature difference will be smaller than the actual. The system will then turn on too late in the morning and turn off too early in the afternoon. Eventually, the system may not even be able to maintain a steady state measured temperature difference greater than TB2 and the pump will than cycle on and off repeatedly during daylight hours, defining another type of failure. Regardless of the direction of the drift of the measured temperatures, the performance of a controller which uses measured temperature differences is not stable nor is it optimal.
2) Another failure mode, but more serious, can occur under freezing conditions. Some control systems circulate the water through the collectors to keep them from freezing. If the drift of the measured collector temperature is positive, the water in the collector can actually freeze before the measured temperature triggers the control system to begin circulation. Once frozen solid, no circulation can occur, even if the pump is turned on. Many of the pumps used in the industry employ the circulating water to keep them from overheating. Since no circulation can occur, the pump can overheat and burnout. Additionally, the collectors can be ruptured by the freezing water. To compound the problem, the temperature sensors for the collectors have the greatest drift potential since their environment is much harsher than that for the reservoir temperature sensors. The most reliable freeze protection system, aside from using glycol as the heat transfer medium, is to use the pump to circulate the water when freezing temperatures are approached within the collector. For this approach to be effective, sensor accuracy is crucial.
3) Sensor malfunctions present another problem to conventional controllers. If a water temperature sensor in a system fails and the control system is using measured temperature differences, the controller can do one of two things. It can turn the pump on continuously, regardless of temperature conditions, or it simply leaves the pump off regardless of conditions. In the first case, the heat gained during daylight hours is lost during twilight hours, resulting in significant heat losses at night. In the later case, no heat is gained. This subjects the collectors to extremely high temperatures during daylight hours, and risks potential freezing conditions at night.